Basically, you are right. But, a factory overclocked card usually came with a better heat dissipation. Thus, gives consumer another degree to squeeze even more cranking on it.
And of course the tested their product before they sell it wouldn't they? For such company to release that kind of product tend to confident that they have made a new standard from the reference card or some sort, but not always be true in some cases. However, they still, most of the times, use a better stuff too aside the heatsink. Like a better PCB, a better design or some sort. And this justifies the price they're asking.